3.7.1 Intrusive investigation methods

There are several methods of excavating into the sub-surface. Many of these methods have been described in great detail in other guidance. A summary is given in the following alineas. An outline of the methods that are applicable to nuclear-licensed sites and defense sites is given in Table 3.43.

Particular reference is made to the specific details that make techniques more or less suitable for use on potentially radioactively contaminated sites. Of particular relevance are excavation techniques that minimise the amount of spoil generated and minimise the potential for contamination to be spread around the excavation area. All of the methods described are technically valid, but their applicability will vary depending on site conditions and on the requirements of the survey.

Technique

Outline of method

Advantages

Disadvantages

Hand-digging

Use of trowel to collect samples to < 0.5m up to hand-dug pits to approximately 1m

* Samples can be collected from any surface location.
* Base of hole can be monitored during excavation.
* Little equipment is required.
* Low potential for contamination to be spread.
* Low risk of damaging services.
* Cheap.

* Only appropriate for fine grained soft sediments.
* Samples are significantly disturbed and there is a high potential for cross contamination of layers.
* Maximum depth of sampling 1-2 m.

Trial pitting

Use of tracked or wheeled excavator to dig trial pit to < 6 m depth

* Large volume of soil exposed.
* Sampling and logging more representative.
* Observations of base of trial pit can be used to identify potential hazards.
* Base of excavation may be monitored for services and contamination as trial pit progresses.

* Small quantities of waste produced.
* Core can be produced in clear plastic sleeves.
* Simple to monitor cores to select samples and for health and safety purposes.
* Relatively quick.
* Cheap.

* Not very reliable in granular soils.
* Samples are usually compacted.
* Small quantities of samples are recovered.
* Samples are not suitable for many geotechnical tests.
* Maximum depth usually < 5 m.
* Possible to use in special restricted areas.
* Difficult to identify water strikes.

* Vibration of drill bit can cause heating of the bit and volatilisation of volatile organics.

Cable percussive in soils/weak rocks

* Suitable for a wide range of materials.
* Suitable for in-situ geotechnical testing and geotechnical sampling.
* Good definition of depth of materials.
* Little or no use of drilling fluid.
* Suitable for the installation of permanent groundwater or gas monitoring installations.
* Possible to use low-head room rigs for sampling in difficult areas.

* Drilling process produces relatively large quantities of spoil (although less than trial pitting).
* Driller’s mate closely involved with drilling process and has relatively high potential to become contaminated.
* Relatively slow.
* Can be regarded as noisy.
* Maximum depth tens of metres depending on material.

* Rapid drilling possible.
* Can be used to drill through overburden using rotary percussive drilling.
* Maximum depth hundreds of metres.
* Good quality core and samples.
* Suitable for the installation of permanent groundwater or gas monitoring installations.

Because trial pits generate large quantities of spoil, their use should be minimised in areas known to be radioactively contaminated. Key aspects to be considered by intrusive investigation methods are:

Field logging.

Minimising cross contamination.

Backfilling with and disposal of soil.

Development pumping.

Radiological pumping.

Radiological clearance of equipment.

These are discussed in the following sections.

3.7.1.1 Borehole drilling

While investigating contaminated areas one of the main objectives will be to ensure the acquisition of an undisturbed sample, preferably with a 100% recovery rate. When samples may be taken using drilling equipment, caution must be taken that cross contamination of samples below more active strata does not take place. This can occur if activity is carried on the coring bit or if cutting fluids are used during the operation. The influence of cross-contamination on individual samples can be reduced if the outer layer of the core sample is carefully removed before analysis takes place.

Once a core has been recovered it is important to carefully cut open the liner and expose the undisturbed core on a work bench. This should then be photographed, logged and sampled at a constant frequency (0.5 m may suffice in short length cores, although it may be appropriate to analyze at closer intervals if, say, the contamination is believed to have leached downwards from the surface and is concentrated near to the top layer of soil) and, in addition, at any particular features of interest. It is often advisable to confirm the size of the required sample with the laboratory and ensure that a duplicate sample is taken.

3.7.1.2 Trial pitting and trenching

Trial pits and trenches are often used as a relatively cheap yet quick method of viewing and sampling the subsurface strata. Stratigraphic and structural changes can be seen more clearly than in cored material and samples are easy to obtain. The approximate maximum depth of 4 m is one of the disadvantages of trenching. Sample points at one-half meter intervals are normally sufficient for contaminant analysis, and once the sample has been obtained the procedures prior to laboratory analysis are similar to that for cores. When done with care, trenching can be used to obtain subsurface samples free of cross-contamination but it is labour intensive and may be unacceptable for environmental or safety reasons. Trenching may generate unacceptable quantities of waste and may expose workers to both physical hazards from unstable ground formations as well as high levels of radiation from the exposed surface.

3.7.1.3 Cone Penetrometer or Direct-Push Technology

Cone penetrometer testing (CPT) or, more generally, direct-push technology provides an opportunity for subsurface measurement without coring or boring. It depends on hydraulically pushing a small-diameter instrumented probe from the ground surface downward. Depending on the soil conditions and size of the pushing device, the depth of penetration can reach tens of meters.

CPT probes include a variety of sensors to identify different contaminants. They are often used to screen contaminated areas for later placement of monitoring wells. Sensors for radioactivity are presently under development and in testing.
The primary advantages of direct push technology over boring are small disturbances, relatively rapid sampling, low cost, and no creation of waste. The limitations are requirements for site access for the truck-mounted device, resistance of some lithologies to penetration, and semi-quantitative nature of the measurements from present sensors.

3.7.1.4 Field logging

It is important to log all relevant information when carrying out an intrusive investigation. Such information should consist of, as a minimum: